Motion sensors and accelerometers are widely used in VCR cameras and aerospace and automotive safety control systems and navigational systems, such as crash sensing systems. Examples of automotive applications include anti-lock braking systems, active suspension systems, supplemental inflatable restraint systems such as air bags, and seat belt lock-up systems.
An example of a motion sensor employed in automotive systems is the yaw rate sensor, which senses movement of an automobile about a vertical axis through its center of gravity. Alternatively, an accelerometer measures acceleration, or more accurately, the force that is exerted by a body as the result of a change in the velocity of the body. Both types of sensors operate on the basis of a moving body possessing inertia which tends to resist a change in velocity.
In the past, electromechanical and electronic motion sensors have been widely used in the automotive industry to detect an automobile's deceleration. More recently, sensors which employ a plated metal surface micromachine have been developed which can be integrated with CMOS circuits on a wafer. As is known by those skilled in the art, plated metal surface micromachines are formed by a metal plating technique in cooperation with a mold which defines the shape of the micromachine on the surface of a wafer. Copending U.S. patent application Ser. No. 08/055,880 to Putty et al. discloses a novel motion sensor of this type which employs a unique resonating metal ring and spring system as a proof mass.
A yaw rate motion sensor 10 of the type taught by Putty et al. is illustrated in FIGS. 1 and 2. As shown, the sensor 10 includes a metal ring 12 which is supported by a number of arcuate springs 14 extending from a center post 16. Circumscribing the ring 12 is an electrode pattern composed of a number of individual electrodes 18. With this construction, the sensor 10 is able to detect rotary movement about the vertical axis through the center post 16 and, therefore, rotary movement about the vertical axis of the automobile. In operation, some of the electrodes 18 are energized to drive the ring 12 into resonance, others are energized to balance the resonant peaks of the rotary movement by inducing stiffness in the ring 12 and springs 14, while the remaining electrodes 18 are used to sense rotary motion of the ring 12. Accelerometers can also be fabricated based on this type of sensor construction.
FIG. 2 illustrates the construction of the sensor 10 on a silicon substrate 20. The sensor 10 primarily relies on a pair of metal layers 22 and 24 which serve as electrical interconnects for the balance and drive electrodes 18. These metal layers 22 and 24 are generally aluminum, deposited on a field oxide layer 42 to electrically isolate the layers 22 and 24 from the substrate 20, and are coated with an oxide or nitride film as an interlevel dielectric layer 26 and a passivation layer 28. Holes 30 are etched in the dielectric layer 26 to allow the metal layers 22 and 24 to be interconnected.
FIG. 3 represents the layout of the first metal layer 22, which forms a concentric conductor pattern composed of individual concentric conductors 32. FIG. 4 represents the layout of the second metal layer 24, which forms a radial conductor pattern composed of individual radial conductors 34, each of which terminate at its radially outward end with an enlarged pad 36. The radial conductors 34 electrically interconnect the concentric conductors 32 with the electrodes 18.
Also shown in FIG. 2 is a third metal layer 38, which the sensor 10 requires as an electrical interconnect for the ring 12, while also forming a bias plane 40 under the ring 12 and as a plating seed layer 44 for forming the ring 12, springs 14, post 16, and electrodes 18. This third metal layer 38 is typically a multilayer of a tungsten-silicon alloy and gold, or a titanium-tungsten alloy and gold, although it is foreseeable that other suitable materials could possibly be used, and is deposited on the passivation layer 28 so as to be directly beneath the ring 12 and springs 14.
The layout of the third metal layer 38 is shown in FIG. 5, illustrating the bias plane 40 and the plating seed layer 44. The bias plane 40 serves to prevent the ring 12 from being deflected towards or away from the substrate 20, which is critical for all capacitive and resonating micromachine devices. In that the ring 12 must be biased from about 5 to about 100 volts to induce resonance, and the substrate 20 is maintained at ground potential, the ring 12 is electrostatically attracted to the substrate 20. Therefore, the bias plane 40 is held at roughly the same potential as the ring 12, so as to prevent the ring 12 from being deflected toward the substrate 20.
The ring 12, spring 14, post 16 and electrodes 18 are then formed by first depositing and patterning a sacrificial layer (not shown) on the third metal layer 38 and the passivation layer 28. Openings are then formed through the sacrificial layer and down to the third metal layer 38 so as to form a mold for the metal micromachine structures. A plating technique is then performed to form the ring 12, spring 14, post 16 and electrodes 18 illustrated in FIGS. 1 and 2.
Sensors 10 of this type are capable of extremely precise measurements, and are therefore desirable for use in automotive applications. However, the intricate structures required to form these sensors 10 must be precisely formed in order to ensure the proper operation of the sensors 10. In particular, the concentric and radial conductors 32 and 34 must be precisely formed without metal stringers being formed between the individual conductors 32 and 34 of each layer. Stringers tend to form during the metal deposition process, and may lead to shorts between electrodes 18 and between the electrodes 18 and the bias plane 40. The tendency for stringer formation is sufficiently high to cause greatly reduced die yield for the manufacturing process. The use of the metal layer 24 to form the radial conductors 34 of the sensor 10 requires dielectric planarization of the metal layer 24, to reduce the tendency for stringer formation, which raises processing costs. As known to those skilled in the art, conventional planarizing techniques include the use of spin-on glass (SOG), etch-back or polyimide layers to form a more planar upper surface on a deposited layer, thereby eliminating notches and surface irregularities on the surface of the deposited layer.
An additional potential shortcoming of the sensor 10 is that the bias plane 40 lies directly under the electrodes 18 and ring 12, such that the tendency for shorting is significantly increased, which also may reduce process yield. Finally, the implementation of all three metal layers 22, 24 and 38 is not fully compatible with MOS, CMOS and BICMOS processes, in that additional masking levels are required to form three metal layers, resulting in higher fabrication costs.
Therefore, while the teachings of Putty et al. provide a precision motion sensor 10 which is highly suitable for automotive applications, it would be highly desirable if further reductions in manufacturing complexity and costs could be achieved. In particular, it would be desirable if such a sensor 10 could be fully integrated into MOS, CMOS and BICMOS processes to improve the efficiency of its fabrication, and thereby reduce production costs and time, while simultaneously improving production yield by reducing the tendency for electrical shorting between individual metal layers.
Therefore, it would be advantageous to provide a plated metal surface micromachine motion sensor whose construction is not only able to accurately detect motion and acceleration of a body, but also requires a minimal number of processing steps so as to facilitate its manufacture and reduce production costs, while also increasing reliability and production yields.